1. Technical Field of the Invention
The present invention relates generally to communication systems, and more particularly to high-speed serial bit stream communications.
2. Description of Related Art
The structure and operation of communication systems is generally well known. Communication systems support the transfer of information from one location to another location. Early examples of communication systems included the telegraph and the public switch telephone network (PSTN). When initially constructed, the PSTN was a circuit switched network that supported only analog voice communications. As the PSTN advanced in its structure and operation, it supported digital communications. The Internet is a more recently developed communication system that supports digital communications. As contrasted to the PSTN, the Internet is a packet switch network.
The Internet consists of a plurality of switch hubs and digital communication lines that interconnect the switch hubs. Many of the digital communication lines of the Internet are serviced via fiber optic cables (media). Fiber optic media supports high-speed communications and provides substantial bandwidth, as compared to copper media. At the switch hubs, switching equipment is used to switch data communications between digital communication lines. WANs, Internet service providers (ISPs), and various other networks access the Internet at these switch hubs. This structure is not unique to the Internet, however. Portions of the PSTN, wireless cellular network infrastructure, Wide Area Networks (WANs), and other communication systems also employ this same structure.
The switch hubs employ switches to route incoming traffic and outgoing traffic. A typical switch located at a switch hub includes a housing having a plurality of slots that are designed to receive Printed Circuit Boards (PCBs) upon which integrated circuits and various media connectors are mounted. The PCBs removably mount within the racks of the housing and typically communicate with one another via a back plane of the housing. Each PCB typically includes at least two media connectors that couple the PCB to a pair of optical cables and/or copper media. The optical and/or copper media serves to couple the PCB to other PCBs located in the same geographic area or to other PCBs located at another geographic area.
For example, a switch that services a building in a large city couples via fiber media to switches mounted in other buildings within the city and switches located in other cities and even in other countries. Typically, Application Specific Integrated Circuits (ASICs) mounted upon the PCBs of the housing. These ASICs perform switching operations for the data that is received on the coupled media and transmitted on the coupled media. The coupled media typically terminates in a receptacle and transceiver circuitry coupled thereto performs signal conversion operations. In most installations, the media (e.g. optical media), operates in a simplex fashion. In such case, one optical media carries incoming data (RX data) to the PCB while another optical media carries outgoing data (TX data) from the PCB. Thus, the transceiver circuitry typically includes incoming circuitry and outgoing circuitry, each of which couples to a media connector on a first side and communicatively couples to the ASIC on a second side. The ASIC may also couple to a back plane interface that allows the ASIC to communicate with other ASICs located in the enclosure via a back plane connection. The ASIC is designed and implemented to provide desired switching operations. The operation of such enclosures and the PCBs mounted therein is generally known.
The conversion of information from the optical media or copper media to a signal that may be received by the ASIC and vice versa requires satisfaction of a number of requirements. First, the coupled physical media has particular RX signal requirements and TX signal requirements. These requirements must be met at the boundary of the connector to the physical media. Further, the ASIC has its own unique RX and TX signal requirements. These requirements must be met at the ASIC interface. Thus, the transceiver circuit that resides between the physical media and the ASIC must satisfy all of these requirements.
Various standardized interfaces have been employed to couple the transceiver circuit to the ASIC. These standardized interfaces include the XAUI interface, the Xenpak interface, the GBIC interface, the XGMII interface, and the SFI-5 interface, among others. The SFI-5 interface, for example, includes 16 data lines, each of which supports a serial bit stream having a nominal bit rate of 2.5 Giga bits-per-second (GBPS). Line interfaces also have their own operational characteristics. Particular high-speed line interfaces are the OC-768 interface and the SEL-768 interface. Each of these interfaces provides a high-speed serial interface operating at a nominal bit rate of 40 GBPS.
Particular difficulties arise in converting data between the 40×1 GBPS line interface and the 16×2.5 GBPS communication ASIC interface. In particular, operation on the 40 GBPS side requires the ability to switch data at a very high bit rate, e.g., exceeding the bit rate possible with a CMOS integrated circuit formed of Silicon. While other materials, e.g., Indium-Phosphate and Silicon-Germanium provide higher switching rates than do Silicon based devices, they are very expensive and difficult to manufacture. Further, the functional requirements of interfacing the 40×1 GBPS line interface and the 16×2.5 GBPS communication ASIC interface are substantial. Thus, even if a device were manufactured that could perform such interfacing operations, the effective yield in an Indium-Phosphate or Silicon-Germanium process would be very low.
Given the drive to achieve greater and greater levels of integration, it would be desirable to integrate at least the silicon portion of the transceiver circuit within the ASIC circuit itself. This would eliminate the standard interface between the ASIC and the transceiver circuit, as well as the need for PC board interconnect lines between the ASIC and the transceiver circuit. This would require the transceiver circuit to operate at a first clock frequency domain to interface with the data being received at the 40×1 GBPS line interface, while the ASIC runs at a second clock frequency domain which is several times lower.
For the transceiver circuit to communicate effectively with the ASIC, notwithstanding that they are running at significantly different clock speeds, typically a circuit such as a DLL is employed to lock the two clocks together in phase such that the ASIC clock is capable of capturing data transmitted to it from the transceiver circuit. To adjust the phase to eliminate any skew between the two clocks, a phase interpolator is often used in the DLL to provide the necessary phase adjustment. Phase interpolators, however, can be complex, dissipate large amounts of power and be somewhat difficult to program.
Thus, there is a need in the art for a simpler, lower power interpolator solution that is capable of providing fine degradations of phase for controlling the lock between a high-speed and low-speed line interface. There is also a need to lock two low speed clocks using a high-speed clock to eliminate skew between the low-speed clocks.